The invention relates to a soundproofing material made of nonwoven materials containing thermoplastic fibres for the acoustic frequency range of 100 to 5000 Hz. Also disclosed is a method of using the soundproofing material in secondary soundproofing.
Many acoustic problems cannot be solved satisfactorily merely by using primary soundproofing measures which are applied to a sound source, and additional secondary measures are required. Secondary measures are those which, as a rule, intervene in the transmission path of the acoustic energy. Either the energy is reflected, that is to say deflected, or the energy is converted into a different energy form, mostly heat. In the first case, insulation is used, and in the latter case, the sound is attenuated.
The prior art in conventional sound insulation uses secondary reduction measures at some distance from the source by disposing reflecting walls into the propagation path of the acoustic energy. Examples are cellular walls, partition walls or acoustic screens.
Also, in conventional sound attenuation, the prior art methods convert the acoustic energy in the medium frequency to high-frequency range into heat through the use of porous sound absorbers, wherein the extent of conversion depends on the frequency range of the sound. For example, artificial mineral fibres, open-cell foamed materials, porous inorganic bulk materials or natural fibres are used. In order to avoid abrasion of the materials, and to prevent them from escaping, they are often laminated with pourable protective materials based on a nonwoven textile.
The fact that porous absorbers are generally tried and tested only in the medium to high-frequency range is based on their physical attenuation properties. In order to attenuate an acoustic wave with the highest possible absorption, the thickness of the attenuating material must be at least one quarter of the wavelength xcex to be attenuated, since the displacement range of a sound wave is the greatest over that range. Therefore, low frequencies determine the required thickness of insulating material due to their longer wavelength. This effect can also be achieved by means of thinner material thicknesses in combination with an air gap. The insulating material is in this case arranged at a distance corresponding to xcex/4. However, degree of absorption of airborne sound xcex1 describing the attenuation capacity is in such a configuration marked by dips in the higher-frequency range.
A significant requirement for secondary soundproofing materials, in particular in spatial acoustics, is the lowest possible insulating material thickness, in order to lose as little spatial volume as possible. In the case of these absorbers, even at a thickness of 10 cm, a distinct reduction in the absorption properties below about 800 Hz is observed. In order to achieve broadband absorption properties, even down to the low-frequency range, absorbers are used in combination with resonators which, on the basis of oscillation processes, withdraw energy over a narrow band from the acoustic wave at a resonant frequency. Their effect is primarily observed in the lower frequency range.
Since secondary soundproofing primarily concerns combating noise in the frequency range of about 200 to 4000 Hz, it is generally the case that neither porous absorbers nor resonators are on their own able to achieve efficient, broadband sound attenuation over the entire frequency range of interest. However, the possible combinations of the two types of soundproofing take up a great deal of space and are expensive.
The applications for nonwoven materials in soundproofing vary. Nonwoven materials often are used in combination with other flat materials or as supports for sound-absorbing materials. Pure nonwoven materials in needled form have been investigated for sound absorption by P. Banks-Lee, H. Peng and A. L. Diggs (TAPPI Proceedings 1992 Nonwovens Conference, pp. 209-216). It was established that, in spite of variations in various trial parameters, the nonwoven materials exhibit a sound absorption which is only insufficient for practical use in the frequency range of  less than 1000 Hz.
EP 0 607 946 contains a description of pure nonwoven materials with thermoplastic fibres as a sound-insulating material. Table 2 of the reference shows that the absorption values in the lower frequency range are at a level which is inadequate for practical use.
The invention has the objective of providing a soundproofing material which, in addition to having a low requirement for space, exhibits a broadband absorption in the frequency range from 100 to 5000 Hz.
According to the invention, this object is achieved by a nonwoven material containing thermoplastic fibres being permanently compacted to a specific flow resistance of RS=800-1400 Ns/m3 in two stages by a mechanical compaction process and a subsequent pressure/heat treatment.